This invention relates to a gas flow measuring apparatus, and particularly to an air flow measuring apparatus for measuring the amount of flow of air which an engine, for example, sucks.
There has hitherto been known from, for example, U.S. Pat. No. 4,089,214, an apparatus of this type which comprises a gas flow measuring tube provided in a suction duct of, for example, an automobile engine, an electric heater of a platinum resistance wire provided in the gas flow measuring tube, and temperature dependent resistors respectively provided on the upstream side and downstream side of the electric heater in the gas flow measuring tube, whereby output signals produced from the electric heater and temperature dependent resistors are used to measure the flow of intake air (gas to be measured).
This conventional apparatus has an advantage that the amount of air flow can be measured with a small-sized and simple arrangement. However, this apparatus employs the electric heater and temperature dependent resistors which are each formed of a very fine platinum resistance wire so supported as to float in the flow of intake gas, so that if the platinum resistance wire undergoes mechanical shocks due to back-fire (a phenomenon in which explosive combustion of a mixture of fuel and air in the suction duct is caused by spreading fire originating from abnormal combustion in the combustion chamber when timings of ignition and intake/exhaust valve switching are immaturely adjusted in the engine combustion system), the electric heater and temperature dependent resistors tend to be damaged, thus impairing durability of the apparatus.
The conventional apparatus also includes a resistor wire grid provided at the outlet of the gas flow measuring tube, for protecting the electric heater and temperature dependent resistors from impulsive pressure and high temperature resulting from the back-fire. The resistor wire grid, however, degrades efficiency of suction of air to the engine.